Plasma etching, deposition, and other processing techniques using a magnetron to contain the plasma above a substrate are well known to those having skill in the microelectronic device fabrication art. In a typical magnetron, magnetic confinement of a low pressure radio frequency (RF) ionic discharge is used to generate a high density plasma in order to expose a substrate to an ionic flux. As is well known to those having skill in the art, a magnetron may be employed to increase the ionic flux density, at a given plasma sheath voltage (defined below), to produce an anisotropic (directional) etch of a pattern into a substrate resulting in minimal undercutting and minimal unwanted enlargement of the etch pattern. The plasma sheath voltage is the electric potential that develops in the area between the substrate and the plasma. Electrons are largely excluded from this area due to the force exerted on the electrons by the electric field.
Magnetrons may also be used to deposit materials onto a substrate by exposing a material to be deposited to the high ionic flux. The substrate is placed outside of the region of intense flux such that the atoms and molecules ejected from the target material by the ionic flux, condense upon the substrate to be processed.
In order to efficiently process a substrate at a high rate without causing unwanted damage to the substrate, it is important that a high density plasma be developed, at low plasma sheath voltage. A high plasma density is necessary so that large numbers of ionic species can strike the substrate to process the substrate at an acceptable rate of production. Low sheath voltage is required so that the energy of the impinging ions is sufficiently low to restrict the effects of the impinging ions close to the substrate surface where material removal (etching) or build up (deposition) occurs. Higher energy ions impinge the surface and distribute excess energy to a greater depth in the substrate. This excess energy is ineffective for the etching or deposition processes and is undesirable since it results in the production of unwanted heat and substrate damage. Even in cases where the chemical activation energy for etching of the substrate being processed is typically less than one electron volt (eV), conventional etching devices must use energies on the order of many hundreds of eV in order to have sufficient ion current across the plasma sheath for useful etch rates. This higher ion energy produces unwanted heat and substrate damage.
The magnetron configuration attempts to obtain high ionic density at a low plasma sheath voltage by using a magnetic field to increase the density of electrons that cause ionizing collisions in the region above the substrate to be processed. In a typical magnetron, a magnetic field (B) is produced parallel to the substrate surface. This parallel magnetic field reduces the mobility of electrons to the surface of the substrate. An electric field (E)is produced perpendicular to the substrate surface (and therefore perpendicular to the magnetic field) by energizing a cathode below the substrate, thereby creating a plasma sheath. The combined effect of the E and B fields produces an electron drift velocity described by the cross product of the electric field and magnetic field vectors (E.times.B). Accordingly, the region in which the electrons are confined, and therefore the ionic concentration is greatest, is known as the E.times.B drift region.
A remote plasma source or generator is desirable to increase plasma density in the E.times.B region, thereby increasing the processing rate. With a remote plasma source, the cathode requires a lower power input to create a large flux of ions to impinge the substrate surface. The cathode need only be biased to create an electric field perpendicular to the substrate surface in order to create an E.times.B electron drift region. The sheath voltage, and hence the electric field in the region, may be independently adjusted to produce the desired ion flux energy for a particular processing operation.
The art has attempted to couple remote sources to a substrate surface through various techniques. In U.S. Pat. No. 4,738,761, to Bobbio et al., and assigned to the assignee of the present invention, coupling of the plasma between the split cathode source and the substrate is accomplished through a continuous sheath voltage region. Therefore, the E.times.B electron drift velocity allows the electrons to move in a continuous closed path above the substrate surface and below the cathode surface, thereby creating a high density plasma in the region above the substrate.
In U.S. Pat. No. 4,588,490 to Cuoumo et al., the remote source penetrates the wall of the chamber and is disposed above and to one side of the magnetron target. The plasma stream emanating from the remote source is transported to the substrate surface across magnetic field lines.
Notwithstanding the above described attempts to improve magnetron performance, and in particular to improve coupling of remote plasma sources, present magnetrons are still limited as to the ionic flux density which can be achieved for a given energy imparted from the ionic flux to the substrate surface. Accordingly, high energy plasma must be used to achieve useful etch or deposition rates, thereby resulting in substrate damage and other unwanted effects, or slower processing rates must be tolerated to avoid substrate damage.